feat(clock): Clock Prescale (CLKPR) support #68

close #68

2 files changed, 184 insertions, 0 deletions

diff --git a/src/peripherals/clock.spec.ts b/src/peripherals/clock.spec.ts
new file mode 100644
index 0000000..69fc832
--- /dev/null
+++ b/src/peripherals/clock.spec.ts
@@ -0,0 +1,92 @@
+import { CPU } from '../cpu/cpu';
+import { AVRClock, clockConfig } from './clock';
+
+// Clock Registers
+const CLKPC = 0x61;
+
+// Register bit names
+const CLKPCE = 128;
+
+describe('Clock', () => {
+  it('should set the prescaler when double-writing CLKPC', () => {
+    const cpu = new CPU(new Uint16Array(0x1000));
+    const clock = new AVRClock(cpu, 16e6, clockConfig);
+    cpu.writeData(CLKPC, CLKPCE);
+    cpu.writeData(CLKPC, 3); // Divide by 8 (2^3)
+    expect(clock.frequency).toEqual(2e6); // 2MHz
+    expect(cpu.readData(CLKPC)).toEqual(3);
+  });
+
+  it('should not update the prescaler if CLKPCE was not set CLKPC', () => {
+    const cpu = new CPU(new Uint16Array(0x1000));
+    const clock = new AVRClock(cpu, 16e6, clockConfig);
+    cpu.writeData(CLKPC, 3); // Divide by 8 (2^3)
+    expect(clock.frequency).toEqual(16e6); // still 16MHz
+    expect(cpu.readData(CLKPC)).toEqual(0);
+  });
+
+  it('should not update the prescaler if more than 4 cycles passed since setting CLKPCE', () => {
+    const cpu = new CPU(new Uint16Array(0x1000));
+    const clock = new AVRClock(cpu, 16e6, clockConfig);
+    cpu.writeData(CLKPC, CLKPCE);
+    cpu.cycles += 6;
+    cpu.writeData(CLKPC, 3); // Divide by 8 (2^3)
+    expect(clock.frequency).toEqual(16e6); // still 16MHz
+    expect(cpu.readData(CLKPC)).toEqual(0);
+  });
+
+  describe('prescaler property', () => {
+    it('should return the current prescaler value', () => {
+      const cpu = new CPU(new Uint16Array(0x1000));
+      const clock = new AVRClock(cpu, 16e6, clockConfig);
+      cpu.writeData(CLKPC, CLKPCE);
+      cpu.writeData(CLKPC, 5); // Divide by 32 (2^5)
+      cpu.cycles = 16e6;
+      expect(clock.prescaler).toEqual(32);
+    });
+  });
+
+  describe('time properties', () => {
+    it('should return current number of microseconds, derived from base freq + prescaler', () => {
+      const cpu = new CPU(new Uint16Array(0x1000));
+      const clock = new AVRClock(cpu, 16e6, clockConfig);
+      cpu.writeData(CLKPC, CLKPCE);
+      cpu.writeData(CLKPC, 2); // Divide by 4 (2^2)
+      cpu.cycles = 16e6;
+      expect(clock.timeMillis).toEqual(4000); // 4 seconds
+    });
+
+    it('should return current number of milliseconds, derived from base freq + prescaler', () => {
+      const cpu = new CPU(new Uint16Array(0x1000));
+      const clock = new AVRClock(cpu, 16e6, clockConfig);
+      cpu.writeData(CLKPC, CLKPCE);
+      cpu.writeData(CLKPC, 2); // Divide by 4 (2^2)
+      cpu.cycles = 16e6;
+      expect(clock.timeMicros).toEqual(4e6); // 4 seconds
+    });
+
+    it('should return current number of nanoseconds, derived from base freq + prescaler', () => {
+      const cpu = new CPU(new Uint16Array(0x1000));
+      const clock = new AVRClock(cpu, 16e6, clockConfig);
+      cpu.writeData(CLKPC, CLKPCE);
+      cpu.writeData(CLKPC, 2); // Divide by 4 (2^2)
+      cpu.cycles = 16e6;
+      expect(clock.timeNanos).toEqual(4e9); // 4 seconds
+    });
+
+    it('should correctly calculate time when changing the prescale value at runtime', () => {
+      const cpu = new CPU(new Uint16Array(0x1000));
+      const clock = new AVRClock(cpu, 16e6, clockConfig);
+      cpu.cycles = 16e6; // run 1 second at 16MHz
+      cpu.writeData(CLKPC, CLKPCE);
+      cpu.writeData(CLKPC, 2); // Divide by 4 (2^2)
+      cpu.cycles += 2 * 4e6; // run 2 more seconds at 4MhZ
+      expect(clock.timeMillis).toEqual(3000); // 3 seconds in total
+
+      cpu.writeData(CLKPC, CLKPCE);
+      cpu.writeData(CLKPC, 1); // Divide by 2 (2^1)
+      cpu.cycles += 0.5 * 8e6; // run 0.5 more seconds at 8MhZ
+      expect(clock.timeMillis).toEqual(3500); // 3.5 seconds in total
+    });
+  });
+});
diff --git a/src/peripherals/clock.ts b/src/peripherals/clock.ts
new file mode 100644
index 0000000..1dbf4ba
--- /dev/null
+++ b/src/peripherals/clock.ts
@@ -0,0 +1,92 @@
+/**
+ * AVR8 Clock
+ * Part of AVR8js
+ * Reference: http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf
+ *
+ * Copyright (C) 2020, Uri Shaked
+ */
+
+import { CPU } from '../cpu/cpu';
+import { u32, u8 } from '../types';
+
+const CLKPCE = 128;
+
+export interface AVRClockConfig {
+  CLKPR: u8;
+}
+
+export const clockConfig: AVRClockConfig = {
+  CLKPR: 0x61,
+};
+
+const prescalers = [
+  1,
+  2,
+  4,
+  8,
+  16,
+  32,
+  64,
+  128,
+  256,
+
+  // The following values are "reserved" according to the datasheet, so we measured
+  // with a scope to figure them out (on ATmega328p)
+  2,
+  4,
+  8,
+  16,
+  32,
+  64,
+  128,
+];
+
+export class AVRClock {
+  private clockEnabledCycles = 0;
+  private prescalerValue = 1;
+  cyclesDelta = 0;
+
+  constructor(
+    private cpu: CPU,
+    private baseFreqHz: u32,
+    private config: AVRClockConfig = clockConfig
+  ) {
+    this.cpu.writeHooks[this.config.CLKPR] = (clkpr) => {
+      if ((!this.clockEnabledCycles || this.clockEnabledCycles < cpu.cycles) && clkpr === CLKPCE) {
+        this.clockEnabledCycles = this.cpu.cycles + 4;
+      } else if (this.clockEnabledCycles && this.clockEnabledCycles >= cpu.cycles) {
+        this.clockEnabledCycles = 0;
+        const index = clkpr & 0xf;
+        const oldPrescaler = this.prescalerValue;
+        this.prescalerValue = prescalers[index];
+        this.cpu.data[this.config.CLKPR] = index;
+        if (oldPrescaler !== this.prescalerValue) {
+          this.cyclesDelta =
+            (cpu.cycles + this.cyclesDelta) * (oldPrescaler / this.prescalerValue) - cpu.cycles;
+        }
+      }
+
+      return true;
+    };
+  }
+
+  get frequency() {
+    return this.baseFreqHz / this.prescalerValue;
+  }
+
+  get prescaler() {
+    return this.prescalerValue;
+  }
+
+  get timeNanos() {
+    return ((this.cpu.cycles + this.cyclesDelta) / this.frequency) * 1e9;
+  }
+
+  get timeMicros() {
+    return ((this.cpu.cycles + this.cyclesDelta) / this.frequency) * 1e6;
+  }
+
+  get timeMillis() {
+    return ((this.cpu.cycles + this.cyclesDelta) / this.frequency) * 1e3;
+  }
+}